new-school type mismatch label can be overly long

[arielb1]

Meta

$ rustc -V
rustc 1.12.0-dev (d5b9850d7 2016-07-16)

STR

fn main() {
    let y = Some(Err(()));
    let x: Option<Option<_>> = y;
}

Result

$ RUST_NEW_ERROR_FORMAT=y rustc fail.rs
error: mismatched types [--explain E0308]
 --> fail.rs:3:32
  |>
3 |>     let x: Option<Option<_>> = y;
  |>                                ^ expected enum `std::option::Option`, found enum `std::result::Result`
note: expected type `std::option::Option<std::option::Option<_>>`
note:    found type `std::option::Option<std::result::Result<_, ()>>`
error: aborting due to previous error

Observe the deeply nested label containing both "expected" and "found".

[estebank]

Re: #21934, #43354 & #40186.

[estebank]

Current output:

error[E0308]: mismatched types
 --> src/main.rs:5:32
  |
5 |     let x: Option<Option<_>> = y;
  |            -----------------   ^ expected enum `Option`, found enum `Result`
  |            |
  |            expected due to this
  |
  = note: expected enum `Option<Option<_>>`
             found enum `Option<Result<_, ()>>`
help: try wrapping the expression in `Some`
  |
5 |     let x: Option<Option<_>> = Some(y);
  |                                +++++ +

I believe that the other open tickets linked above are better for tracking the outstanding work needed to improve these errors.